Add a `versions` endpoint on RPC services

It'd be nice to have a versions endpoint on RPC services, which would return the list of versions of the swh components currently used by that instance.

This could be implemented as an endpoint registered by the base RPCServerApp, to which we would pass a list of relevant swh.xxx modules that the RPC service knows it uses.

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Migrated from T4008 (view on Phabricator)